Deer in Mythology

 Celtic

The Celts held deer as supernatural animals, "fairy cattle" that were herded and milked by a localised and benevolent fairy giantess (a bean sìdhe) in each district, who could shift shape to that of a red deer; in the West Highlands, she selected the individual deer that would be slain in the next day's hunt.
In Ireland, An Chailleach Bhéarach, "The Old Woman of Beare", an island off the coast of County Cork, takes the form of a deer to avoid capture; to Beare come characters from the Land of the Dead to visit Ireland. Other Celtic mythological figures such as Oisin, Flidais and Sadb were given connections to deer.
Cernunnos was a god in Celtic mythology that possessed two deer antlers on the top of his head. He was known as The Horned One or The Horned God despite having antlers and not horns. Cernunnos is also known as The Stag Lord, The Horned God of the Hunt, The Lord of the Forest, The Lord of the Hunt, and The Lord of the Animals.
The deer was said to be a fairy creature that could pass between the worlds. This was especially true for a white deer. Fionn's wife Sabha became a deer when she went to the Otherworld. Beautiful women frequently became deer in many tales while fleeing from hunters.

 Christianity

In the story of Saint Hubertus, on Good Friday morning, when the faithful were crowding the churches, Hubertus sallied forth to the chase. As he was pursuing a magnificent stag the animal turned and, as the pious legend narrates, he was astounded at perceiving a crucifix standing between its antlers, which occasioned the change of heart that led him to a saintly life. The story of the hart appears first in one of the later legendary hagiographies (Bibliotheca hagiographica Latina, nos. 3994–4002) and has been appropriated from the earelier legend of Saint Eustace (Placidus).
Later in the 6th century, the Bishop Saint Gregory of Tours wrote his Chronicles about the Merovingian rulers, were appeared a Legend of the King Clovis I who prayed to Christ in one of his campaigns so he could find a place to cross the river Vienne. Considered as a divine sign, a huge deer appeared and showed were could the army pass across.
In the 14th century, probably keeping some relation with Saint Eustace's legend, the deer again appears in the Christian Legends. The Chronicon Pictum contains a legend, where the later King Saint Ladislaus I of Hungary and his brother the King Géza I of Hungary were hunting in a forest and appeared to them a deer with numerous candles on his antlers. As the saint Knight said to his brother, that wasn't a deer but an angel of God, and his antlers were wings, the candles were shining feathers. And as Ladislaus added, the place where the deer was standing was where it was meant to be built a cathedral in honor of the Holy Virgin.